1. Field of the Invention:
The present invention relates to an apparatus for preparing a sample solution for an infrared spectrophotometer, in particular a solution from a liquid chromatograph, and a method of analyzing the sample solution.
2. Prior Art:
Apparatuses for measuring the infrared spectra of components separated by a liquid chromatograph, by using an infrared spectrophotometer disposed at the detection section of the liquid chromatograph, generally employ a flow cell (References: (1) C. C. Johnson & L. T. Taylor, Anal. Chem., 55, 436 (1983), (2) R. S. Brown, Anal. Chem., 55, 1492 (1983), (3) C. C. Johnson & L. T. Taylor, Anal. Chem., 56, 2642 (1984), and (4) L. T. Taylor, el. al., Anal. Chem., 59, 295 (1987)). In such an apparatus, a column effluent is led into the flow cell, infrared radiation is irradiated onto this flow cell, and the intensities of the infrared radiation which has been transmitted through the flow cell are measured by an infrared spectrophotometer to obtain its spectra.
This apparatus enables continuous and automatic measurement of the infrared spectra.
However, it has a disadvantage in that any kind of solvent employed in this apparatus also absorbs radiation in the infrared region. This limits the wavelength regions and the types of solvent that can be used in the measurement. Further, the material of the window of the flow cell is restricted in terms of its transmittance of infrared radiation, thereby limiting the wavelengths that can be used in the measurement.
In order to obviate these problems, a method of measuring an infrared spectrum has been proposed in which a sample solution is allowed to drip onto a sample retaining board made of crystals of KBr which transmits infrared radiation, a solvent in each droplet is evaporated to deposit a solute, and infrared radiation is transmitted through the solute to measure an infrared spectrum (Reference: K. Jinno & C. Fujimoto, HRC & CC., 4, 532 (1981)).
In this method, however, the sample solution inevitably expands on the retaining board. It is therefore necessary to greatly reduce the flow rate of the mobile phase in the liquid chromatograph, thereby limiting the amount of sample which can be used in the measurement of the infrared spectra. Further, KBr is a deliquescent substance, so it must be preserved in a dry atmosphere. If a retaining board made of KBr is employed, a solvent like water cannot be used as the mobile phase. KBr absorbs radiation in the far infrared region, so the KBr retaining board cannot be used for the measurement of a sample which shows absorption in the far infrared region (the wavenumber region of 400 cm.sup.-1 or less). The size of this sample retaining board is limited in terms of its manufacture, handling, and preservation. This prevents continuous, automatic measurement of the infrared spectra over a long period of time (for example, for an entire day).
A diffuse reflectance method has also been known, in which diffuse reflection cups used for retaining samples, and which contain NaCl, KBr, of CaF.sub.2, are aligned on a conveyor, the column effluent is sequentially allowed to drip into each of these cups, and a solvent of each droplet of sample solution is evaporated to deposit a solute (Reference: D. T. Kuehl & P. R. Griffitho, Anal, Chem., 52, 1394 (1980)).
This method requires the use of NaCl, CaF.sub.2, or other substances, and the maximum number of cups which can be prepared at one time is limited to about 50. Therefore, the infrared spectra cannot be measured continuously and automatically over a long period of time.